A rotatable connector device includes a fixed-side member, a rotatable-side member rotatably attached to the fixed-side member, and a flat cable accommodated in an annular accommodation space formed therebetween. One of two ends of the flat cable is fixed to the fixed-side member and is connected to a connector, and the other end of the flat cable is fixed to the rotatable-side member and is connected to a connector. An intermediate part of the flat cable in a longitudinal direction thereof is a folded part that is curved and folded. Owing to this structure, regardless of whether the direction in which the rotatable-side member is rotated is clockwise or counterclockwise, the flat cable is wound and rewound with resilience, which allows signal transmission.
In the case where such a rotatable connector device is used for multiple high-speed communication of a CAN (Controller Area Network) or the like, there are problems that the electromagnetic compatibility is decreased due to electromagnetic noise and that the impedance is unstabilized.
In order to improve the electromagnetic compatibility, Patent Document 1 proposes a flat cable having a twisted pair structure, which is obtained as follows. A flat cable including a pair of signal conductors is folded along many folding lines which are parallel to each other and angled with respect to a width direction thereof.
However, the twisted pair structure spoils the resilience of the flat cable and thus inhibits smooth rotation of the rotatable-side member.
In order to decrease the noise and solve the problem of unstable impedance, a method for applying a shield layer to a flat cable including a signal transmission path has been proposed (see Patent Document 2).
According to one specific method therefor, a shield conductor and a signal conductor acting as a signal transmission path are provided in one covered cable. However, this method has the following problem. Usually, a flat cable is formed of two, namely, top and bottom, resin laminate films by thermally fusing adhesive layers attached thereto. Therefore, it is difficult to accurately keep the distance between the signal conductor and the shield conductor. In addition, sliding or curving of the flat cable that accompanies the rotation of the rotatable-side member causes buckling or abrasion. This results in a shield layer of the shield conductor being delaminated or cracked, which decreases the electromagnetic compatibility due to electromagnetic noise and unstabilizes the impedance.
In order to solve these problems, it is conceivable to provide a spacer sheet between the signal conductor and the shield conductor so as to fuse and connect these conductors while an appropriate distance is kept therebetween. However, this is not preferable because it is difficult to keep the adhesiveness between the spacer and the conductors, and also because the use of the spacer raises the cost.
It is also conceivable to attach a conductive material to an outer surface of the flat cable including the signal conductor by use of electroless plating or the like. However, this method has an undesirable possibility that sliding or bending of the flat cable causes bucking or abrasion, which causes the conductive material to be delaminated or cracked.